The storage and transportation of a wide variety of goods is greatly facilitated by the use of pallets. Pallets allow the storage and movement of different items by a common material handling system employing forklift trucks. In the early years of pallet usage, most pallets were constructed of hardwoods because of its low cost, ready availability and high compressive strength.
Wood pallets are still widely used in the industry. However, wood pallets are subject to splintering, moisture absorption, and the steel fasteners which hold wooden pallets together will rust if exposed to water. Plastic pallets are advantageously used where cleanliness, repeated usage or special attachment needs are presented.
All general purpose pallets share several basic structural properties. They have a generally flat upper deck for supporting boxes, canisters or crates, and they have two or more openings for the admittance of fork lift tines. The most universally useful pallet will allow the fork lift tines to enter from all four sides of the pallet. The tine openings may be formed either between a pallet top deck and a pallet bottom deck, or the pallet may have only a single deck with an array of legs which support the deck above a support surface to allow entrance of fork lift tines beneath the deck.
Many manufacturing processes have been adapted to production of plastic pallets: injection molding, cellular foam, blow molding, and rotomolding. However, the large size of pallets, often four feet long or greater, makes the thermoforming process particularly well suited to the production of pallets.
U.S. Pat. No. 4,428,306 to Dresen et al. discloses a pallet produced in a twin-sheet thermoforming process in which the upper sheet is fused to the lower sheet in the walls of downwardly protruding cup-like feet.
In the thermoforming process a sheet of thermoplastic material is heated until it becomes soft and moldable, but not fluid. The heated sheet is held against a mold, whereupon a vacuum is drawn between the mold and the plastic sheet, drawing the sheet down onto the mold, and causing the thermoplastic sheet to conform to the mold's surface. In twin-sheet thermoforming both an upper sheet and a lower sheet are heated and molded simultaneously in two separate molds. The heated sheets are then pressed together within the molds. The effect is to create an article which may have enclosed volumes, and regions of plastic of desired thicknesses.
A key element of the further utilization of plastic pallets is making the pallet competitive with low cost hardwood pallets. A significant portion of the cost of any plastic pallet, especially those produced in large quantities, is the raw material cost of the plastic resin and extruded sheet from which it is fabricated. Hence, the watchword of plastic pallet design is structural efficiency. A high structural stiffness per pound of plastic will yield an economically competitive pallet.
A pallet manufactured by Penda Corporation in the 1980's employed a significant advance in twin-sheet thermoforming structures. This pallet utilized adjacent narrow protruding ribs on one mold half which depressed one heated sheet to fuse to the other. However, the ribs were sufficiently close together that not only did the deformed sheet fuse to the opposite sheet, it also fused to itself at the base of the neighboring rib. These vertical fusions or "webs" provided vertically extending regions of solid plastic which gave pallet designers a valuable tool in increasing structure stiffness.
Pallets can be loaded in a variety of ways, depending on whether the pallet is supported on its legs, on a rack, or on the tines of a fork lift. Many approaches to achieving sufficient deck thickness have been employed, for example by utilizing upper sheet channels which are fused to lower sheet channels which run perpendicular to the upper channels. Despite past successes, economics and competitive pressures drive the need for plastic pallets of ever greater stiffness and load supporting capability at ever-reduced weights.